The ever decreasing size and increased complexity of current devices lead to certain consequences for an inbuilt transducer. To optimize the ratio between space needed inside the device and sound-emanating area, speakers are more and more rectangular or oval instead of circular for example. Whereas circular speakers are fully symmetrical, rectangular and ovals speakers comprise some asymmetries which in turn lead to poor sound quality, which is to improved.
FIGS. 1a and 1b show a first (left half) and a second (right half) embodiment of a rectangular prior art speaker 1 with rounded corners, FIG. 1a in top view, FIG. 1b in a cross-sectional view. Speaker 1 comprises a membrane 2, a coil 3 attached to said membrane 2, a magnet system 4 interacting with coil 3 and a housing 5 for carrying aforesaid parts. The membrane 2 of the second embodiment additionally comprises corrugations 6.
The membrane 2 is divided into a first area A1, a second area A2, which is arranged for translatory movement in relation to said first area A1, and a third area A3, which connects said first A1 and said second area A2. Furthermore, a closed line L is shown, which is arranged within said third area A3 and encompasses said second area A2. As said line L is parallel to the outer border of the rectangular speaker 1 with rounded corners or the identically shaped membrane 2 respectively, it comprises four straight sections a with four curved sections b in-between. Furthermore, two directions are shown in FIGS. 1a and 1b. First, a direction of translatory movement DM, which is parallel to the axis of the speaker 1 and which indicates the direction of movement of said second area A2. Second, a direction DL of said line L, which is obvious for the straight sections a and which is the tangent to said line L in the curved sections b. Line direction DL and translatory movement direction DM are perpendicular to each other in each point of said line L. FIGS. 1a and 1b only show 2 examples of such pairs, one situated in a straight section a and one in a curved section b (not shown in FIG. 1b).
The first area A1 in the present example is the border of the membrane 2, which is connected to the housing 5 and therefore immovable with respect to the housing 5. Said second area A2 is the area inside the outer border of coil 3 in the present example. Second area A2 therefore covers the joint face between coil 3 and membrane 2 as well as the so-called dome. Said second area A2 may translatorily move in relation to first area A1. Other movements, which occur in a real and thus non-ideal speaker, such as rocking, bending and a certain side movement are disregarded for the further considerations. Second area A2 is therefore considered to move as a whole, which means that it does not change its shape.
Third area A3 now connects said first A1 and said second area A2. Since said second area A2 moves in relation to said first area A1, said third area A3 changes its shape. In the straight sections a there is a simple rolling movement, which means that there are no movements in line direction DL inside the membrane 2. A completely different situation exists in the curved sections b. Here a movement of the membrane 2 in translatory movement direction DM causes a relative movement in line direction DL inside the membrane 2. This relative movement is caused by a change of radius of the curved sections b which in turn is caused by the translatory movement of second area A2.
The problem addressed is well known in the prior art, why usually corrugations 6 as the second embodiment of speaker 1 has are put in the curved sections b so as to allow aforesaid relative movement in line direction DL. The exact physical explanation is, that the planar spring constant psc, which is in line direction DL, has decreased. So normally the planar spring constant psc in a curved section b is lower than in a straight section a. However, it has been found out that simply putting corrugations 6 into curved sections b is not sufficient for a satisfying function of a speaker, which is explained in more detail in the following section.
Reference is therefore made to FIG. 2a, which shows a graph of the planar spring constant psc and the translatory spring constant tsc of aforesaid prior art membranes 2 along a quarter of said line L, hence sweeping half of a straight section a of the long side of membrane 2, a curved section b, and half of a straight section a of the small side of the membrane 2. The planar spring constant psc is in line direction DL and the translatory spring constant tsc is in translatory movement direction DM as mentioned before.
The solid lines show parameters for the first embodiment of the prior art membrane 2 with no corrugations. Here the planar spring constant psc is more or less constant provided that the membrane 2 is homogeneous. As a result, the translatory spring constant tsc is dramatically increased in the corners of the membrane 2 or in the curved sections b respectively which in turn leads to some unwanted consequences:                warping of membrane 2, which in turn leads to distorted sound reproduction as well as to increased local loads on the coil 3. This might damage the coil 3, in particular in case of a so-called self supporting coil;        decreased stroke of membrane 2, which in turn leads to reduced volume or poor efficiency respectively;        local peak loads within membrane 2, which in turn leads to buckling or breaking of membrane 2.        
The dashed lines now show parameters for the membrane 2 having corrugations 6 in the curved sections b. Thus the planar spring constant psc shows a step down in the curved section b. The corrugations 6 are well designed, so that the translatory spring constant tsc in the middle of the curved section b has the same value as in the straight sections a. So one could believe that the problem is solved therewith, which was obviously a doctrine in speaker design. However, there is an unpredictable rise and drop in the graph of the translatory spring constant tsc at the border between the straight sections a and curved sections b, which again leads to the addressed consequences. This is because of the interaction between the straight sections a and curved sections b. If the third area A3 is theoretically split into separate straight sections a and curved sections b, the associated deformations will be different when the second area A2 moves. But because the straight sections a and the curved sections b are interconnected at their edges, said interaction and in turn an influence of the translatory spring constant tsc occur. More recent investigations have revealed this unwanted effect.
It should be noted that there are some further embodiments of prior art membranes comprising complex structures of bulges and corrugations in different embodiments, which are difficult to manufacture and which do not sufficiently solve the objects addressed above either.